1. Field of the Invention
The present invention relates to image forming apparatuses that have a function of forming an image on a sheet and are, for instance, copiers and printers.
2. Description of the Related Art
In recent years, image forming apparatuses, such as printers and facsimile machines, have become inexpensive, and widely been used in offices and stores for advertisement and in-house production of catalogs. Along therewith, there are increasing needs not only for high image quality but also for high medium flexibility supporting various types of sheets and for reduction in size to allow installation in a small office and store with a compact space.
Coated sheets (high quality paper sheets onto which gloss coating is applied) have a particularly high need among paper sheets. The coated sheets are characterized by shine, high smoothness and capability of clear reproduction of photographs and characters. Accordingly, the coated sheets are suitable for advertisement and catalogs. Unfortunately, the coated sheets have a problem in that, if a stack of paper is left in a high humidity environment, a surface layer of paper absorbs moisture and is easy to adhere to a paper sheet in contact. Thus, in the conventional type of image forming apparatuses, there is a problem to be solved of double feeding, wherein both the first sheet (hereinafter, referred to as “preceding sheet”) conveyed prior to others and the second sheet (hereinafter, referred to as “following sheet”) adhered on the first sheet and conveyed after the first sheet are doubly conveyed together.
To address the problem, as disclosed in Japanese Patent Application Laid-Open No. H11-157686, a technique has been proposed that blows air on the sides and top surface of paper sheets stacked on a sheet stack portion to improve sheet separation.
However, there is a case of causing double feeding of coated sheets owing not only to adhesion in a high humidity environment by absorbed moisture but also to adhesion by an electrostatic force. More specifically, in a small image forming apparatus configured such that a preceding sheet in secondary transfer overlaps and is in contact with the following sheet on a sheet stack portion, the preceding and following sheets adhere to each other owing to an electrostatic force in the following cases. That is, the case where a user touches the following sheet with a bare hand, and the case where, even if without touching, material configuring a stack portion has a small resistance and the sheet stack portion is electrically grounded.
It is considered that the double feeding is caused by adhesion due to an electrostatic force unless the sheets are always separated from each other by a sufficient distance during transfer. Thus, separation due to air as disclosed in Japanese Patent Application Laid-Open No. 11-157686 is not a highly reliable measure.